(1) Field of the Invention
The present invention relates to a technique of applying a newly found bonding molecule, capable of bonding tightly to the surface of a metal, to the surface treatment of metal blanks. More specifically, the present invention provides a method for forming on the surface of a metal a covering layer having a thickness of the angstrom order, which is much smaller than the thickness of a covering layer formed by a conventional so-called adhesive primer.
It was found that if a thermoplastic resin layer is coated on the metal surface treated with this bonding molecule, the initial adhesion and the adhesion after the lapse of time are highly improved over these adhesions attainable by the conventional methods. Even if a lacquer containing a thermosetting resin instead of the thermoplastic resin is coated and dried on the metal surface treated with this bonding molecule, a similar effect can be attained. Accordingly, when an organic material is bonded or laminated to the so-treated metal surface or when metal-to-metal bonding is performed, a strong bonding can be maintained in the bonded portion.
(2) Description of the Prior Art
A desire to form a strong bonding by heating between a metal and a thermoplastic resin layer is very strong in various fields.
For example, in the field of the can-manufacturing industry, there is broadly adopted a method for forming a can body by lapping both the end portions of a metal blank for a can body through a thermoplastic adhesive such as a polyamide adhesive and heat-bonding the lapped end portions. When metal blanks are thus bonded together through a thermoplastic adhesive, the bonding strength between the surface of the metal blank and the thermoplastic adhesive is not always high even just after the bonding operation and the bonding strength tends to decrease with the lapse of time. As means for eliminating this defect, there is ordinarily adopted a method in which an adhesive primer such as an epoxy-phenolic resin is coated on a metal blank and is baked and a thermoplastic resin adhesive is heat-bonded through this adhesive primer layer.
However, the epoxy-phenolic lacquer is relatively expensive and the operation of forming a coating layer of this lacquer on the metal blank is troublesome. Moreover, since the lacquer has to be applied in the form of a solution in an organic solvent, the solvent cost and the energy cost for baking become necessary. Furthermore, since discharge of the organic solvent into the open air is not allowed, an environmental pollution-preventing device such as an after-burner should be provided.
Strong bonding between a thermoplastic resin and a metal blank is eagerly desired also in the field of film-laminated steel plates as substitutes for conventional coated steel plates, and also in this case, the above-mentioned defects are similarly observed.